The kinematic envelope of a railway is the amount of space needed around a train to accommodate the bounce and sway of its carriages. Although the sub-surface lines have a larger kinematic profile than the deep Tube lines, experts believe some enlargement work will be needed to accommodate full-gauge main line trains. This is likely to mean grinding the edges off platforms, raising overbridges and moving services attached to the sides of tunnels. Straighter bends may also be needed to accommodate longer carriages.
Links between the main line railway and Underground would have to be built to connect the Great Western Line to the Circle Line at Paddington and the East London Line South to Croydon at New Cross Gate. In engineering terms this would be relatively straightforward since Tube and rail lines in these areas run close together and are within Railtrack or London Underground owned land. But changing track layouts would cause considerable disruption to existing services. Powers exist to allow an extension of the East London Line to Highbury & Islington on the North London Line which would create a connection all the way to the main line at Finsbury Park.
Inter-running of trains
Underground trains are designed to have low top speeds, high acceleration and rapid braking, whereas main line trains have higher top speeds, lower acceleration and slower braking. Inter-running the two types of rolling stock on Underground lines is likely to cause time tabling problems. Main line trains would take longer to travel between the Tube's closely spaced stations. Some rail engineers believe there may be a need for more frequent track maintenance or track upgrading since the heavier main line trains are likely to exert greater forces on the track bed.
Convincing the HM Railways Inspectorate that it is safe to mix main line trains with Underground trains is likely to be difficult. The crash-worthiness of Underground trains would have to be reviewed to see how they would react in a collision with heavier main line trains. Compatible safety systems which prevent trains passing signals set at danger would also have to be introduced and the position of CCTV screens and cameras may have to be changed to allow drivers of both types of train to see when passengers are clear of the doors. A method of safe access and egress of passengers from main line trains which become stuck in tunnels would also have to be worked out since, unlike Underground trains, main line trains are not equipped with emergency doors at either end.
Headways between Underground trains on the Metropolitan Line between Paddington and Liverpool Street are already down to two minutes during peak times. Squeezing more train paths into the timetable would require a sophisticated moving-block signalling system of the type which London Underground has so far failed to install on the Jubilee Line Extension. Even then experts believe that headways could not be reduced to much less than one minute thirty seconds. Signalling sections on the Underground are also much shorter than those on the main line so the two systems would have to be made compatible.
Main line trains coming into Paddington work on an overhead catenary power supply; those coming into Liverpool Street use a third rail system and the Underground trains use a four rail system. London Underground already has ambitions to convert all trains to overhead supplies for safety reasons but this would be difficult, given the limited kinematic envelope. Converting the four rail system on the Underground to a three rail system would mean boosting the positive rail from 420V to 750V and tying the negative rail to earth. Rail engineers believe this would make Tube trains more susceptible to electrical failure.
Engineering standards on the main line and Underground have evolved separately. Engineers from Railtrack and LU will have to work together to develop a new common code of standards appropriate to both forms of transport.
Platforms and stations
Running main line trains on the Underground is likely to mean extending platforms to accommodate longer trains and altering the platform height or width to minimise gaps between doorways and platform edges at particular locations. Stations may also have to be extended and new access facilities constructed to deal with higher passenger volumes. Given that the Underground runs through some of the most sensitive parts of the city it may be difficult to gain powers to increase station capacity.
Structural load bearing capacity
LU has undergone a programme of strengthening priority structures after having to impose speed limits on some stretches of the Tube. Introducing main line trains onto the Tube would mean having to reassess the load bearing capacities and impact resistance for the heavier trains and, where necessary, strengthen them. This is likely to require innovative techniques using high strength materials like carbon fibre.
Given the technical problems, the cost of integrating main line trains with the Underground sub surface lines is likely to be high with few new train paths available. This may mean reducing existing Underground services to accommodate main line services into the city. It is still unclear what the business case for this would be and how many passengers might prefer this service.